1. Field of the Invention
This invention relates to a method of forming an image so that a subtraction image free from noise is obtained in a subtraction processing of radiation images. This invention particularly relates to a method of forming an image so that a subtraction image free from noise is obtained in a digital subtraction processing of radiation images wherein stimulable phosphor sheets are used.
2. Description of the Prior Art
Conventionally, a digital subtraction processing method is used for processing radiation images. In the method, two radiation images recorded under conditions different from each other are photoelectrically read out to obtain digital image signals, which are then subjected to a subtraction processing with respect to the corresponding picture elements of the images, thereby to obtain a difference signal for forming an image of a specific structure contained in the radiation images. The method makes it possible to reproduce a radiation image of only the specific structure by use of the signal thus obtained.
Basically, the subtraction processing is classified into the so-called temporal (time difference) subtraction processing method and the so-called energy subtraction processing method In the former method, the image of a specific structure is extracted by subtracting the image signal of a radiation image obtained without injection of contrast media from the image signal of a radiation image in which the image of the specific structure is enhanced by the injection of contrast media. In the latter method, an object is exposed to radiations having energy distributions different from each other to obtain two radiation images respectively containing the images of a specific structure recorded on the basis of the intrinsic radiation energy absorption characteristics of the specific structure. Then, the image signals of the two radiation images are weighted appropriately when necessary, and subjected to subtraction to extract the image of the specific structure.
Since the subtraction processing is extremely effective for diagnostic purposes in image processings for medical X-ray photographs, it has recently attracted much attention, and research has continued to develop improved methods by use of electronic technology. The processing technique is specifically called the digital subtraction processing method.
A novel digital subtraction processing method has been proposed, for example, in Japanese Unexamined Patent Publication No. 58(1983)-163340. The method comprises the steps of (i) using two or more stimulable phosphor sheets exhibiting an extremely wide latitude of exposure to a radiation, (ii) exposing the stimulable phosphor sheets to the radiation passing through the same object under different conditions to have radiation images of the object stored in the stimulable phosphor sheets, image information of the specific structure being different between the radiation images, (iii) detecting the radiation images by scanning the stimulable phosphor sheets with stimulating rays to obtain digital image signals, and (iv) conducting a digital subtraction processing by use of the digital image signals. The stimulable phosphor sheets comprise a stimulable phosphor which is able to store a part of the radiation energy when exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays of ultraviolet rays, and then emits light in proportion to the stored energy of the radiation when exposed to stimulating rays such as visible light, as disclosed for example in U.S. Pat. No. 4,258,264. The stimulable phosphor sheets exhibit an extremely wide latitude of exposure and a markedly high resolving power. Therefore, when the digital subtraction processing is conducted by use of the radiation images stored in the stimulable phosphor sheets, it is possible to obtain a radiation image having an improved image quality, particularly a high diagnostic efficiency and accuracy, regardless of the amount of radiation to which the stimulable phosphor sheets are exposed.
As one of the methods of obtaining two original images subjected to the energy subtraction processing conducted by use of the stimulable phosphor sheets, there has heretofore been known a method wherein the stimulable phosphor sheets are changed quickly at the image recording position and are exposed to a radiation of high energy and, approximately at the same timing, the object is exposed to a radiation of high energy and a radiation of low energy which are switched over quickly, so that radiation images of the object corresponding to these radiations are stored in the respective stimulable phosphor sheets. For simplicity, this method is hereinafter called the double exposure method. However, when the double exposure method is conducted, the radiation of high energy readily passes through the stimulable phosphor sheet, and the amount of radiation energy absorbed by the stimulable phosphor sheet becomes small. Therefore, the radiation image (high energy image) obtained with the radiation of high energy contains a high level of noise, and a subtraction image obtained thereby contains much noise and exhibits a low image quality, particularly a low diagnostic efficiency and accuracy.